Surge-protected fuse switch



Sept. 26, 1939. R. R. PlTTMAN El AL SURGE-PROTECTED FUSE SWITCH Filed Sept. 15, 1958 a r w W J 1|! l 6 2 J1 2 a W 5 E w W m a 9 a w W an F Z 3 5 T. 4 7 z m 2 my. a MWwMwWr J w M ,7 M v Q l/duWW/W WW WWW- w m y 9 H/w i 9 4. 1N 0 x W a a m 3 5 0 0 0? w a I? 0 E 2 W 0 a 2 l? Patented Sept. 26, 1939 UNITED STATES SURGE-PROTECTED FUSE SWITCH Ralph R. Pittman and Carroll H. Walsh, Pine Blufl, Ark.

Application September 15, 1938, Serial No. 230,076

14 Claims. (Cl. 200115) This invention relates to fuse switches particularly to high voltage fuse switches commonly employed for connecting electrical apparatus, such as transformers or capacitors, to distribution circults.

Among the objects of the invention may be noted the provision of means for shunt-circuiting the fusible element of the fuse switch under a high superimposed and rapidly changing current in the circuit of which the fuse switch is a part, which may be caused by atmospheric electrical discharges, such shunt-circuiting preventing the passage of the high current through the fusible element, and thereby avoiding rupture ofthe element and consequent interruption to service; the provision of a fuse switch embodying means for selecting, in accordance with rate of change of current, high current surges for by-passing around the fusible element; and means for effecting the desired by-pass result without affecting the desired protective features of the fuse switch under abnormal current having normal rate of change of current. Other objects will be in part obvious, and in part pointed out hereinafter.

The invention comprises the elements and combinations thereof, features of construction, and arrangement of parts to be exemplified in the description to follow, and the scope of the invention will be indicated in the accompanying claims. In the drawing: Fig. 1 is a side elevational view of the movable member of the device, shown partly in section; Fig. 2 is a side elevational view of the switch in the service position; Fig. 3 is an enlarged section of the dielectric element embodied in the switch; and Fig. 4 illustrates schematically the circuit diagram of the invention.

Similar reference characters indicate corresponding parts in the several figures of the drawing.

It has long been recognized that small loads supplied from high voltage distribution circuits require fractional ampere fuse links to properly protect the connected apparatus; also that such sizes of fuse links are susceptible to blowing due to passage of superimposed surge current of atmospheric origin which are of a transient nature and have no harmful effect upon the connected apparatus. The expense of replacing fuse links so blown, and the undesirability of the attendant interruption to service, have heretofore been well recognized, but no solution has been presented for the problem.

From experience with various sizes of metallic per switch contact 15 is secured to the outer end fusible elements, we have found that a conventional fusible element rated at 10 amperes, which melts at 20 amperes in about 10 seconds, has adequate capacity for carrying non-destructive surge currents without fusing, while sizes below this 5 value become increasingly susceptible to melting from surge currents as the rating is decreased; The surge'carrying capacity of such a 10 amperelink is of the order of 6000 amperes when subjected to a surge current increasing to crest in about 1 microseconds, and decreasing to half value in 40 microseconds. Surge currents of this order, caused by lightning, disturbances, often pass through fusible elements to connected apparatus, as a distribution transformer or capacitor, in charging the capacitance formed by the insulated conducting members forming parts of the apparatus, thereby blowing smaller capacity fuses while affording no necessary protection to the apparatus. 20

On the other hand, should the disturbances be sufficiently severe to cause failure of the insulation of the connected apparatus, the dynamic or follow current must be quickly interrupted to disconnect the faulty apparatus. In addition, any 25 change in the normal operation of the fuse under overload is not desired. The manner in which our invention meets the foregoing requirements will be clear from the following description.

Referring first to Fig. 1 and Fig. 2 of the drawing, a pair of outstanding insulators II and I2 are secured, in any suitable manner, to the respective ends of the base member Ill. The upof the upper insulator II, and electrically connected through the line terminal l4 to the line conductor I3. Similarly, the lower switch contact I6 is secured to the outer end of the lower insulator I2, and electrically connected through 40 the load terminal 11 to the load conductor I8.

The removable member for electrically connecting the line conductor l3 to the load conductor I8 is illustrated in Fig. l. A fuse oartridge 23 is fitted at the upper end thereof with 45 the internally threaded terminal 24 for receiving the button 2! of the fuse link 29, and for ease in manipulating the fuse cartridge, is provided with the eye 25. A screw plug26 is arranged to tightly engage the button 21 to assure 50 electrical contact between these members.

An impedance member 48 is mounted on thefuse cartridge 23 adjacent the upper end thereof, being supported at its lower end by the conducting sup port 33, one portion of the latter encircling the of which is wrapped the helical coil 35, the lower.

end 34 of the latter being electrically connected to the conducting support 33, and the upper end 36 of the coil 35 being electrically connected to a forwardly extending portion of the upper fuse cartridge terminal 24, the latter encircling the tubular insulator ill, and being secured thereto by the fastening pin 52. For weatherproofing, an outer sleeve of insulating material ift is fitted over the coil 35, and a downwardly extending flange 4i pressed over the top of the sleeve 3b. At the upper end of the tubular insulator 6i], and in spaced insulated relation to the upper fuse cartridge terminal Ed, is provided an upper contact member 358, the latter being adapted to frictionally engage the upper switch contact E5. The contact member 38 is maintained in position by means of its internal threaded engagement with the conducting bolt 39, the lower end of the latter being in electrical contact with the conducting support 33. w

The lower end of the fuse cartridge 23 is fitted with the lower fuse contact member 49, which is arranged to engage the lower switch contact it. For guiding the movable member, a pair of laterally extending pins 50 are provided for rotating in the hooks I9. An outwardly extending arm of conducting material 56 is secured and electrically connected to the lower fuse contact member 49 by the mounting screw 58, and the lower end of the fuse link 29 electrically connected to the member 56 at the terminal 54. Preferably, a fuse ejector is provided, including the arm 53, rotatable about the pin 51 at the outer end of the arm 56, and the ejector spring 52, fastened at one end to an upwardly extending portion of the arm 53, and at the other to a portion 5| of the arm 56. -An enlarged portion 55 of the rotatable arm 53 engages the fuse link 29 at a point near the lower end of the fuse cartridge 23, and the spring 52 biases the arm 53 in a direction to withdraw the fuse link from the cartridge.

A fusible element 28 forms a part of the fuse link 29, and in accordance with common practice, is positioned near the top of the fuse cartridge 23. In addition, a dielectric element 30, respon- T; sive to a predetermined voltage, is combined with the fuse link 29 at a point adjacent to the conducting supporting member 33. As shown in Fig. 3, the dielectric element 30 includes a pair of endwise spaced tubes of insulating material 44 and 46, preferably of mica, which are fitted tightly over the fuse link 29. A plurality of circumferentially arranged resilient conducting members 43 extend longitudinally over the space 41 between the adjacent ends of the tubes 44 and 46, and then upwardly and outwardlly, being held in position by means of the outer sleeve 45, which is conveniently of fiber. The latter is tightly pressed over the surface of members 43, which electrically contact the conducting pins 3| and 32 when the fuse link 29 is in the normal position in the fuse tube 23.

From the arrangement just described, it may be observed that the space 41 forms a discharge gap, with the members 43 as one electrode'at the potential of the lower end of the coil 35, and with the fuse link 29 as the other electrode at the arrears potential of the lower end of the fusible element 28.

To further illustrate the organization, in Fig. 2 a transformer or capacitor 2!!! is shown connected to the load conductor it through an insulating entrance bushing iii, the transformer or capaci-- tor tank being connected to the ground 22 in accordance with common practice. In addition, Fig. 4 illustrates schematically the manner in which the arrangement appears to a rapidly changing electrical condition, such as might be caused by lightning. The connected apparatus appears as a capacitance 59, due to the capacity to ground of the. insulated conducting elements forming parts thereof.

The current path through the switch under normal condition is from the line conductor it through the terminal it to the upper switch con-- tact I15, thence to the upper contact member 38, thence through the conducting bolt 39 to the con-- ducting support 33, thence to the lower end of the coil of the impedance member 48, thence through the coil 35 to the conducting member, thence through the upper fuse cartridge terminal 26 to the button 2] of the fuse link 2Q, thence downwardly through the fusible element 28 past the dielectric element 30 to the lower fuse terminal 5d, thence through the conducting support 56 to the lower fuse cartridge contact 49, and thence to the lower switch contact l 6 through the lower terminal ill to the load conductor 58.

As schematically shown in Fig. l, the dielectric element on under normal conditions is not conducting, and therefore no current flows through it in shunt with the series arrangement of the impedance element 48 and the fusible element 28 under normal circuit conditions.

Upon the occurrence of an atmospheric discharge near the conductor 3, a voltage may he suddenly impressed thereon, and a resultant current surge which is rapidly changing in amplitude flows through the series arrangement of the im pedance member 48 and the fusible element 28 to charge the capacitance schematically shown at 59. At the same time the voltage across the coil 35 increases as a function of the rate of change of current, (L di/dt, L representing the inductance of the coil, and di/dt the rate of change of cur- .rent). Before the surge current reaches a value suflicient to fuse the fusible element 28, the voltage across the series arrangement of the impedancemember 48 and the fusible element 28 exceeds the break down value of the dielectric element 30, the electrical dimensions of the elements being purposely made such as to accomplish this end. When the gap 41 of the dielectric element 30 breaks down, the surge current is obviously by-passed around the fusible element 28, to thereby prevent its fusing. As soon as the disturbing condition has passed, the dielectric element resumes its normal dielectric condition, and the fusible element again carries the normal load current.

It might appear, in the event of an insulation failure due to an excessive voltage impressed on the transformer 20, that the normal frequency current would continue to flow through the gap already broken down, and the protective function of the fuse entirely lost. This, however, does not occur, because under the relatively slower rate of change of normal current, the arc voltage of the gap is made sufficient to cause ample current to pass through the fusible element to melt it. It is impedance of the impedance member 48 is great enough to prevent fusing .of the fusible element.

Fbr best operation, the break-down value of the dielectric element should be as low as possible, and at the same time provide a definite open circuit following the passage .of surge current. Also the reactance of the impedance member 48 should be as large as possible without having an excessive value at normal frequency, as the latter condition not only might limit the current to a value below the melting current of the fuse under an arcing condition of the dielectric element at normal frequency, but also might result in excessive voltage drop at normal frequency in the circuit to connected apparatus.

From the above considerations, it is apparent that, for best operation, it is desirable to properly balance the important characteristics of the device; that is, the inductance of the impedance member, the breakdown voltage of the dielectric element, and the electrical energy expressed as heat, which is required to melt thefusible element. These items, being mutually dependent,

should exist in proper relationship. To illustrate, one satisfactory organization is that in which the inductance of the impedance member is such as to result in an impedance of about 0.5 ohm at 60 cycles frequency, the breakdown voltage of the associated gap about 1500 volts, and the capacity of the fusible element 10 amperes r. m; s. for 20 seconds.

To assure, under normal conditions, a definite open circuit in the auxiliary path shunting the fusible element, the length of the spark gap between the electrodes of the dielectric element should not be less than about 0.020 inch, corresponding to a breakdown value, under practically all conditions of applied voltage, of about 1500 volts. Closer electrode spacings have been found to be unreliable, due to thermal effects which cause conducting material to bridge the gaps.

It may be mentioned that the blowing of fuses by surge currents of lightning origin in the circuit to connected apparatus does not in any way prevent the imposition of high voltage on connected apparatus, as the surge current necessarily must pass through the fuse before the latter can be blown. Our invention herein is not intended as one for limiting,the voltage which may reach con-- nected apparatus, but simply as one for protecting the fusible element of a fuse switch from melting as a result of transient surge conditions existing in the circuit of which the fuse switch forms a part.

While we have shownand described a specific embodiment of our invention, it will be apparent that changes and modifications may be made therein without departing from the spirit and scope of our invention, and we desire that our invention be limited only as is necessitated by the prior art and, the accompanying claims.

We claim as our invention:

1. In a'fuse switch, insulating means supporting spaced upper and lower conducting contacts, means for removably supporting a fuse cartridge on said insulating means, a fuse link having a fusible element, said fusible element being disposed within said fuse cartridge, an impedance member mounted on said fuse cartridge, means including a serial arrangement of said impedance member and said fusible element for normally connecting said contacts, and normally non-conducting voltage responsive means bridging the serial arrangement, said voltage responsive means being adapted to break down to effect flow of current therethrough in response to a predetermined voltage and to subsequently resume its normal non-conducting condition in response to the disappearance of said predetermined voltage.

2. In a fuse switch, insulating means supporting spaced upper and lower conducting contacts, means for removably supporting a fuse cartridge on said insulating means, a fuse link including a fusible, element, said fusible element being positioned within said fuse cartridge, an impedance member mounted on said cartridge and removable therewith, means including a series circuit arrangement of said fusible element and said impedance member for normally electrically connecting said contacts, and normally non-conducting means responsive to the attainment of a predetermined voltage across said serial arrangement for shunt-circuiting said serial' arrangement, said last-named means resuming the normal non-conducting condition immediately following the disappearance of said predetermined voltage.

3. In a fuse switch, insulating means supporting spaced upper and lower conducting contacts, means for supporting a fuse cartridge on said insulating means, a fuse link including a fusible element, said fusible element being positioned within said fuse cartridge, an impedance member mounted on said fuse cartridge, means including a series circuit arrangement of said fusible element and said impedance member for normally providing a circuit from one contact to the other, and normally non-conducting means responsive to an abnormal predetermined rate of change of current through said circuit for shunt-circuiting said serial arrangement, said last-named means resuming its normal non-conducting condition when the rate of change of current is less than said predetermined rate of change.

4. In a fuse switch, insulating means supporting spaced upper and lower conducting contacts, means for supporting a fuse cartridge on said insulating means, a fuse link including'a fusible element, said fusible element being positioned within said fuse cartridge, an impedance member mounted on said cartridge, means including a series circuit arrangement of said fusible element and said impedance member for electrically connecting said contacts, and a normally non-comducting dielectric element associated with said fuse link, said dielectric element being connected in shunt relation with said series arrangement and being adapted to break down to effect current flow therethrough only during that period when the voltage exceeds a predetermined value.

5. In a fuse switch, insulating means supporting spaced contacts, means for supporting a fuse cartridge on said insulating means, a fuse link including a fusible element, said fusible element being positioned .within said fuse cartridge, an impedance member associated with said switch, means including a series circuit arrangement of said fusible element and said impedance member for normally electrically connecting said contacts, a normally non-conducting dielectric element associated with said fuse link, and means for connecting said dielectric element in shunt-circuit relation with said series circuit arrangement, said dielectric element being adapted to break down to effect current flow therethrough in response to a predetermined voltage and to resume the normally nonconducting condition following the disappear-- ance of said predetermined voltage.

6. In a fuse switch, insulating means supporting spaced conducting contacts, means for supporting a fuse cartridge on said insulating means,

a removable fuse link including a fusible element extending within said cartridge, an impedance member associated with said fuse cartridge, means including a series circuit arrangement of said fusible element and said impedance member for normally electrically connecting said contacts, a dielectric element mounted on and removable with said fuse link, and means for electrically connecting said dielectric element in shunt circuit relation with said series circuit arrangement, said dielectric element being adapted to break down and effect current flow in the shunt circuit in response to a predetermined voltage. V

l. in a fuse switch, insulating means supporting spaced conducting contacts, means supporting a fuse cartridge on said insulating means, a removably mounted fuse link including a fusible element extending within said cartridge, an impedance member mounted on said cartridge externally thereof, a dielectric element carried by said fuse link and positioned within said cartridge, means including a serial arrangement of said impedance member and said fusible element for electrically connecting said contacts, and means including at least one laterally extending conducting member carried by said fuse link for electrically connecting said dielectric element in shunt circuit relation with said serial arrangement.

8. In a fuse switch, a fuse link having a fusible element, an impedance member electrically connected in serial arrangement with said fusible element, and a dielectric element bridging the serial arrangement, said dielectric element being non-conducting-under normal voltage conditions and adapted to break down to effect flow of current in shunt with said serial arrangement in response to a predetermined voltage and to resume a non-conducting state under normal voltage conditions.

9. A fuse comprising a series circuit arrangement including a fusible element and an impedance member, and normally non-conducting voltage responsive means electrically connected in parallel circuit relation with said series circuit arrangement, said voltage responsive means being adapted to spark over in response to the occurrence of an abnormal predetermined voltage to efiect flow of current therethrough, and to be restored to the normally non-conducting condition in response to reoccurrence of normal voltage conditions.

10. A fuse comprising elements constituting a normal current-carrying path including a serial arrangement of a fusible element and an impedance member, and an auxiliary path including a normally non-conducting dielectric element for art/acre shunting said normal current-carrying path, said dielectric element being adapted to break down and establish current flow through said auxiliary path upon the occurrence of predetermined voltages for the period of the existence of said voltages, and to subsequently open-circuit said auxiliary path;

ll. In a fuse switch, a fuse cartridge, 2. fusible element within said cartridge, an impedance member mounted on said cartridge and electrically connected in series circuit relation with said fusible element, said fusible element being responsive to current to fuse, and normally nonconducting voltage responsive means in parallel with the series arrangement of said fusible element and said impedance member, said voltage responsive means under normal voltage conditions preventing diversion of current flow from said fusible element but under abnormal voltage conditions permitting said diversion of current flow until substantially normal voltage conditions are re-established.

12. In a fuse switch, a fuse cartridge, a fusible element within said cartridge and an impedance member serially connected to said fusible element, and a normally non-conducting dielectric element electrically connected in shunt relation with the serial arrangement of said fusible element and said impedance member, the dielectric element being adapted to withstand normal voltages due to normal rates of change of current through said serial arrangement but to break down under abnormal voltages due to abnormal rates of change of current corresponding to a condition in which it is desirable to shunt-circuit and thereby prevent melting of said fusible element.

13. A fuse device comprising conducting elements constituting a normal current-carrying path including a fuse link embodying a fusible element, and means providing an auxiliary path in shunt-circuit relation with said fusible elcment, said last-named means including a discharge path one electrode of which is a portion of said fuse link.

14. A. fuse device comprising conducting elements constituting a normal current-carrying path including a serial arrangement of a fusible element and an impedance member, and elements constituting an auxiliary normally non-currentcarrying path in shunt relation with said serial arrangement, said auxiliary path being characterized by the inclusion therein of a dielectric element which does not break down to efiect flow of current in said auxiliary path until the voltage across said serial arrangement exceeds about volts.

RALPH R. PI'I'IMAN.

CARROLL H. WALSH. 

